A probabilistic approach to modeling microstructural variability and fatigue behavior in ASTM F562 medical grade wire 1
J. Schaffer*1, 2 Fort Wayne Metals Research Products Corporation, USA; 2Purdue University, USA
[email protected]
REFERENCE: Presented May 19th, 2006 at the 9th International Fatigue Congress, Atlanta, USA.
Abstract Several previous studies have demonstrated the importance of constituent inclusion particle distributions on variability in observed fatigue lifetimes. Such models have often concentrated on crack growth behavior in standard test specimens such as single edge notch tension (SENT) geometries. The methodology used here is similar to others in that constituent particles sizes were modelled as initial cracks that were grown to failure using a fracture mechanics approach. Lognormal distributions were fit to inclusion size parameters of 0.178 mm [0.007 in] ASTM F562 alloy medical grade wire. A critical volume approach was used to generate random subsets of inclusion sizes from which particles were chosen with a maximum likelihood of micro-crack generation. These sites were then grown at several stress levels to failure using a plasticity-induced crackclosure model to predict total fatigue life. The resulting lives were found to be in good general agreement with experimentally observed variability in rotary beam fatigue testing. Keywords: crack growth, mp35n, 35n lt, medical wire, probabilistic fatigue Introduction Medical appliances such as coronary lead implants, heart wall repair devices, and other long term implant technologies require the use of high strength, fatigue resistant and biocompatible wire materials to ensure adequate levels of performance. These devices are subsequently expected to endure the harsh biologic environment as long as the patient lives, often requiring the resistance to tens and even hundreds of millions of stress cycles due to patient movement, pulsatile stresses associated with vessel dilation/contraction, and a variety of other factors. A material of particular importance in coronary lead design is ASTM F562 by wt%, nominally a 35Co-35Ni-20Cr-10Mo alloy. ASTM F562 wire is used in low power bradycardia therapy pacing leads as the primary structural as well as electrical connection between the signal generator and target stimulation site and as a silver cored composite wire form in higher power defibrillation leads. The fatigue process and its mechanisms are known to depend largely upon the presence of material inhomogeneities [1-4]. These may include intrinsic defects such as non-metallic inclusions, micropores, and grain boundaries, and extrinsic defects such as surface scratches and corrosion pits. This study focuses on cold drawn ASTM F562 wire, which is typically free of microporosity, possesses excellent corrosion resistance, and is drawn through diamond dies to yield an extremely smooth surface. The primary focus here is the growth of fatigue cracks from non-metallic
inclusion sites contained within 35N LT®1 and MP35N®2 alloy. That is, two variants of specification ASTM F562 of significantly different microcleanliness whose average elemental assays are shown in Table 1. Material Background MP35N was originally designed by SPS technologies as an aerospace fastener material specifically for the space shuttle [5]. The Co-Ni superalloy was designed to possess a high degree of toughness, corrosion resistance, and excellent cryogenic properties. Table 1. Average wt% assay, MP35N and 35N LT
Element Carbon Manganese Silicon Phosphorus Sulfur Chromium Nickel Molybdenum Iron Titanium Boron Cobalt
MP35N 0.010 0.020 0.040 0.002 0.001 20.45 34.79 9.52 0.44 0.70 0.009 Balance
35N LT 0.010 0.060 0.030 0.002 0.001 20.58 34.82 9.51 0.52 0.01 0.010 Balance
1 35N LT® is a registered trademark of Fort Wayne Metals Research Products Corp., IN, USA. 2 MP35N® is a registered trademark of SPS Technologies, Jenkintown, PA.
2 It is this combination of properties in addition to its remarkable resistance to fatigue damage that have bolstered its use in the medical device industry in the past twenty years. This transition to medical service has also made known some weaknesses in the conventional alloy system. At the micro length scales associated with fine medical wire, the intrinsic constituent titanium nitride (TiN) particle content of MP35N has been shown to be a significant detriment to total fatigue life [4]. In the 35N LT alloy system, the small addition of titanium (
